1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same. The present invention is suitably applied to broadcast TV cameras, video cameras, digital still cameras, surveillance cameras and silver-halide film cameras.
2. Description of the Related Art
In recent years, there have been demands for zoom lenses having a wider angle of view, higher zoom ratio and high optical performance for use in image pickup apparatuses such as TV cameras, silver-halide film cameras, digital cameras and video cameras. As a type of zoom lens having a wide angle of view and high zoom ratio, there has been known a positive lead type five-unit zoom lens comprising five lens units including a lens unit having a positive refractive power disposed on the most object side. There is a known five-unit zoom lens of this positive lead type in which the magnification-varying lens units having functions of variator and compensator are constituted by three movable lens units, which move in loci different from each other (Japanese Patent Application Laid-Open No. H08-234105 and No. H01-126614).
The zoom lenses disclosed in Japanese Patent Application Laid-Open No. H08-234105 and No. H01-126614 are each composed of a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a negative refractive power, a fourth lens unit having a positive refractive power and a fifth lens unit having a positive refractive power for imaging. These documents disclose zoom lenses in which the second, third and fourth lens units move during zooming. Japanese Patent Application Laid-Open No. H08-234105 discloses a five-unit zoom lens having a zoom ratio of approximately 2.5 and an angle of view of approximately 30 degrees at the wide-angle end of the zoom range. Japanese Patent Application Laid-Open No. H01-126614 discloses a five-unit zoom lens having a zoom ratio of approximately 5 and an angle of view of approximately 66 degrees at the wide-angle end of the zoom range.
In order for five-unit zoom lenses to achieve high optical performance while having a wider angle of view and higher zoom ratio, it is important that appropriate conditions on movement of the second, third and fourth lens units as magnification-varying lens units during zooming be set and that the first lens unit be designed to have an appropriate refractive power. In addition, it is important that the second and third lens units be designed to have an appropriate combined refractive power and that the locus of movement of the third lens unit during zooming from the wide-angle end to an intermediate focal length position be set appropriately. Inappropriate setting of the above-mentioned factors will lead to difficulties in achieving a zoom lens having high optical performance throughout the whole zoom range while having a wider angle of view and higher zoom ratio.
In the five-unit zoom lens disclosed in Japanese Patent Application Laid-Open No. H08-234105 and No. H01-126614, an increase in the zoom ratio tends to necessitate an increase in the amount of movement of the movable lens units and an increase in the total length of the zoom lens and to lead to an increase in the variation in aberrations with zooming.